Electrophoretic particles, and processes for the production thereof

ABSTRACT

Copper chromite particles can advantageously be used as black particles in electrophoretic media and displays. Preferably, the copper chromite particles are coated with a silica coating and a polymer coating.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.09/140,846, filed Aug. 28, 1998 (now U.S. Pat. No. 6,727,881), which inturn claims benefit of (1) application Ser. No. 60/057,133, filed Aug.28, 1997; (2) application Ser. No. 60/057,716, filed Aug. 28, 1997; (3)application Ser. No. 60/057,799, filed Aug. 28, 1997; (4) applicationSer. No. 60/057,163, filed Aug. 28, 1997; (5) application Ser. No.60/057,122, filed Aug. 28, 1997; (6) application Ser. No. 60/057,798,filed Aug. 28, 1997; (7) U.S. Ser. No. 60/057,118, filed Aug. 28, 1997;(8) application Ser. No. 60/059,543, filed Sep. 19, 1997; (9)application Ser. No. 60/059,3 58, filed Sep. 19, 1997; (10) applicationSer. No. 60/065,630, filed Nov. 18, 1997; (11) application Ser. No.60/065,605, filed Nov. 18, 1997; (12) application Ser. No. 60/065,629,filed Nov. 18, 1997; (13) application Ser. No. 60/066,147, filed Nov.19, 1997; (14) application Ser. No. 60/066,245, filed Nov. 20, 1997;(15) application Ser. No. 60/066,246, filed Nov. 20, 1997; (16)application Ser. No. 60/066,115, filed Nov. 21, 1997; (17) applicationSer. No. 60/066,334, filed Nov. 21, 1997; (18) application Ser. No.60/066,418, filed Nov. 24, 1997; (19) application Ser. No. 60/071,371,filed Jan. 15, 1998; (20) application Ser. No. 60/070,940, filed Jan. 9,1998; (21) application Ser. No. 60/072,390, filed Jan. 9, 1998; (22)application Ser. No. 60/070,939, filed Jan. 9, 1998; (23) applicationSer. No. 60/070,935, filed Jan. 9, 1998; (24) application Ser. No.60/074,454, filed Feb. 12, 1998; (25) application Ser. No. 60/076,955,filed Mar. 5, 1998; (26) application Ser. No. 60/076,959, filed Mar. 5,1998; (27) application Ser. No. 60/076,957, filed Mar. 5, 1998; (28)application Ser. No. 60/076,956, filed Mar. 5, 1998; (29) applicationSer. No. 60/076,978, filed Mar. 5, 1998; (30) application Ser. No.60/078,363, filed Mar. 18, 1998; (31) application Ser. No. 60/08 1,374,filed Apr. 10, 1998; (32) application Ser. No. 60/081,362, filed Apr.10, 1998; (33) application Ser. No. 60/083,252, filed Apr. 27, 1998;(34) application Ser. No. 60/085,096, filed May 12, 1998; (35)application Ser. No. 60/090,223, filed Jun. 22, 1998; (36) applicationSer. No. 60/090,222, filed Jun. 22, 1998; (37) application Ser. No.60/090,232, filed Jun. 22, 1998; (38) application Ser. No. 60/092,046,filed Jul. 8, 1998; (39) application Ser. No. 60/092,050, filed Jul. 8,1998; (40) application Ser. No. 60/092,742, filed Jul. 14, 1998; and(41) application Ser. No. 60/093,689, filed Jul. 22, 1998. Thisapplication is also a continuation-in-part of copending application Ser.No. 10/063,803, filed May 15, 2002 (Publication No. 2002/0185378, nowU.S. Pat. No. 6,822,782), which itself claims benefit of applicationSer. No. 60/291,081, filed May 15, 2001. Finally, this applicationclaims benefit of copending application Ser. No. 60/481,572, filed Oct.28, 2003. The entire contents of all the aforementioned applications,and of all United States Patents, published applications and copendingapplications mentioned below are herein incorporated by reference.

BACKGROUND OF INVENTION

This invention relates to electrophoretic particles (i.e., particles foruse in an electrophoretic medium) and processes for the production ofsuch electrophoretic particles. This invention also relates toelectrophoretic media and displays incorporating such particles. Morespecifically, this invention relates to novel black or dark coloredelectrophoretic particles.

Particle-based electrophoretic displays, in which a plurality of chargedparticles move through a suspending fluid under the influence of anelectric field, have been the subject of intense research anddevelopment for a number of years. Such displays can have attributes ofgood brightness and contrast, wide viewing angles, state bistability,and low power consumption when compared with liquid crystal displays.

The terms “bistable” and “bistability” are used herein in theirconventional meaning in the art to refer to displays comprising displayelements having first and second display states differing in at leastone optical property, and such that after any given element has beendriven, by means of an addressing pulse of finite duration, to assumeeither its first or second display state, after the addressing pulse hasterminated, that state will persist for at least several times, forexample at least four times, the minimum duration of the addressingpulse required to change the state of the display element. The opticalproperty is typically color perceptible to the human eye, but may beanother optical property, such as optical transmission, reflectance,luminescence or, in the case of displays intended for machine reading,pseudo-color in the sense of a change in reflectance of electromagneticwavelengths outside the visible range. It is shown in the U.S. Publishedapplication No. 2002/0180687 that some particle-based electrophoreticdisplays capable of gray scale are stable not only in their extremeblack and white states but also in their intermediate gray states, andthe same is true of some other types of electro-optic displays. Thistype of display is properly called “multi-stable” rather than bistable,although for convenience the term “bistable” may be used herein to coverboth bistable and multi-stable displays.

Nevertheless, problems with the long-term image quality ofelectrophoretic displays have prevented their widespread usage. Forexample, particles that make up electrophoretic displays tend to settle,resulting in inadequate service-life for these displays.

Numerous patents and applications assigned to or in the names of theMassachusetts Institute of Technology (MIT) and E Ink Corporation haverecently been published describing encapsulated electrophoretic media.Such encapsulated media comprise numerous small capsules, each of whichitself comprises an internal phase containing electrophoretically-mobileparticles suspended in a liquid suspension medium, and a capsule wallsurrounding the internal phase. Typically, the capsules are themselvesheld within a polymeric binder to form a coherent layer positionedbetween two electrodes. Encapsulated media of this type are described,for example, in U.S. Pat. Nos. 5,930,026; 5,961,804; 6,017,584;6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851; 6,130,773;6,130,774; 6,172,798; 6,177,921; 6,232,950; 6,249,271; 6,252,564;6,262,706; 6,262,833; 6,300,932; 6,312,304; 6,312,971; 6,323,989;6,327,072; 6,376,828; 6,377,387; 6,392,785; 6,392,786; 6,413,790;6,422,687; 6,445,374; 6,445,489; 6,459,418; 6,473,072; 6,480,182;6,498,114; 6,504,524; 6,506,438; 6,512,354; 6,515,649; 6,518,949;6,521,489; 6,531,997; 6,535,197; 6,538,801; 6,545,291; 6,580,545;6,639,578; 6,652,075; 6,657,772; 6,664,944 and 6,680,725; and U.S.Patent applications Publication Nos. 2002/0019081; 2002/0021270;2002/0053900; 2002/0060321; 2002/0063661; 2002/0063677; 2002/0090980;2002/0106847; 2002/0113770; 2002/0130832; 2002/0131147; 2002/0145792;2002/0171910; 2002/0180687; 2002/0180688; 2002/0185378; 2003/0011560;2003/0011868; 2003/0020844; 2003/0025855; 2003/0034949; 2003/0038755;2003/0053189; 2003/0076573; 2003/0096113; 2003/0102858; 2003/0132908;2003/0137521; 2003/0137717; 2003/0151702; 2003/0189749; 2003/0214695;2003/0214697 and 2003/0222315; and International applicationsPublication Nos. WO 99/67678; WO 00/05704; WO 00/38000; WO 00/38001; WO00/36560; WO 00/67110; WO 00/67327; WO 01/07961; WO 01/08241; and WO03/104884.

Known electrophoretic media, both encapsulated and unencapsulated, canbe divided into two main types, referred to hereinafter for convenienceas “single particle” and “dual particle” respectively. A single particlemedium has only a single type of electrophoretic particle suspended in asuspending medium, at least one optical characteristic of which differsfrom that of the particles. (In referring to a single type of particle,we do not imply that all particles of the type are absolutely identical.For example, provided that all particles of the type possesssubstantially the same optical characteristic and a charge of the samepolarity, considerable variation in parameters such as particle size andelectrophoretic mobility can be tolerated without affecting the utilityof the medium.) When such a medium is placed between a pair ofelectrodes, at least one of which is transparent, depending upon therelative potentials of the two electrodes, the medium can display theoptical characteristic of the particles (when the particles are adjacentthe electrode closer to the observer, hereinafter called the “front”electrode) or the optical characteristic of the suspending medium (whenthe particles are adjacent the electrode remote from the observer,hereinafter called the “rear” electrode (so that the particles arehidden by the suspending medium).

A dual particle medium has two different types of particles differing inat least one optical characteristic and a suspending fluid which may beuncolored or colored, but which is typically uncolored. The two types ofparticles differ in electrophoretic mobility; this difference inmobility may be in polarity (this type may hereinafter be referred to asan “opposite charge dual particle” medium) and/or magnitude. When such adual particle medium is placed between the aforementioned pair ofelectrodes, depending upon the relative potentials of the twoelectrodes, the medium can display the optical characteristic of eitherset of particles, although the exact manner in which this is achieveddiffers depending upon whether the difference in mobility is in polarityor only in magnitude. For ease of illustration, consider anelectrophoretic medium in which one type of particles is black and theother type white. If the two types of particles differ in polarity (if,for example, the black particles are positively charged and the whiteparticles negatively charged), the particles will be attracted to thetwo different electrodes, so that if, for example, the front electrodeis negative relative to the rear electrode, the black particles will beattracted to the front electrode and the white particles to the rearelectrode, so that the medium will appear black to the observer.Conversely, if the front electrode is positive relative to the rearelectrode, the white particles will be attracted to the front electrodeand the black particles to the rear electrode, so that the medium willappear white to the observer.

If the two types of particles have charges of the same polarity, butdiffer in electrophoretic mobility (this type of medium may hereinafterto referred to as a “same polarity dual particle” medium), both types ofparticles will be attracted to the same electrode, but one type willreach the electrode before the other, so that the type facing theobserver differs depending upon the electrode to which the particles areattracted. For example suppose the previous illustration is modified sothat both the black and white particles are positively charged, but theblack particles have the higher electrophoretic mobility. If now thefront electrode is negative relative to the rear electrode, both theblack and white particles will be attracted to the front electrode, butthe black particles, because of their higher mobility will reach itfirst, so that a layer of black particles will coat the front electrodeand the medium will appear black to the observer. Conversely, if thefront electrode is positive relative to the rear electrode, both theblack and white particles will be attracted to the rear electrode, butthe black particles, because of their higher mobility will reach itfirst, so that a layer of black particles will coat the rear electrode,leaving a layer of white particles remote from the rear electrode andfacing the observer, so that the medium will appear white to theobserver: note that this type of dual particle medium requires that thesuspending fluid be sufficiently transparent to allow the layer of whiteparticles remote from the rear electrode to be readily visible to theobserver. Typically, the suspending fluid in such a display is notcolored at all, but some color may be incorporated for the purpose ofcorrecting any undesirable tint in the white particles seentherethrough.

Both single and dual particle electrophoretic displays may be capable ofintermediate gray states having optical characteristics intermediate thetwo extreme optical states already described.

Some of the aforementioned patents and published applications discloseencapsulated electrophoretic media having three or more different typesof particles within each capsule. For purposes of the presentapplication, such multi-particle media are regarded as sub-species ofdual particle media.

Also, many of the aforementioned patents and applications recognize thatthe walls surrounding the discrete microcapsules in an encapsulatedelectrophoretic medium could be replaced by a continuous phase, thusproducing a so-called polymer-dispersed electrophoretic display, inwhich the electrophoretic medium comprises a plurality of discretedroplets of an electrophoretic fluid and a continuous phase of apolymeric material, and that the discrete droplets of electrophoreticfluid within such a polymer-dispersed electrophoretic display may beregarded as capsules or microcapsules even though no discrete capsulemembrane is associated with each individual droplet; see for example,the aforementioned 2002/0131147. Accordingly, for purposes of thepresent application, such polymer-dispersed electrophoretic media areregarded as sub-species of encapsulated electrophoretic media.

A related type of electrophoretic display is a so-called “microcellelectrophoretic display”. In a microcell electrophoretic display, thecharged particles and the suspending fluid are not encapsulated withinmicrocapsules but instead are retained within a plurality of cavitiesformed within a carrier medium, typically a polymeric film. See, forexample, International Application Publication No. WO 02/01281, andpublished U.S. application No. 2002/0075556, both assigned to SipixImaging, Inc.

Although electrophoretic media are often opaque (since, for example, inmany electrophoretic media, the particles substantially blocktransmission of visible light through the display) and operate in areflective mode, many electrophoretic displays can be made to operate ina so-called “shutter mode” in which one display state is substantiallyopaque and one is light-transmissive. See, for example, theaforementioned U.S. Pat. Nos. 6,130,774 and 6,172,798, and U.S. Pat.Nos. 5,872,552; 6,144,361; 6,271,823; 6,225,971; and 6,184,856.Dielectrophoretic displays, which are similar to electrophoreticdisplays but rely upon variations in electric field strength, canoperate in a similar mode; see U.S. Pat. No. 4,418,346.

An encapsulated or microcell electrophoretic display typically does notsuffer from the clustering and settling failure mode of traditionalelectrophoretic devices and provides further advantages, such as theability to print or coat the display on a wide variety of flexible andrigid substrates. (Use of the word “printing” is intended to include allforms of printing and coating, including, but without limitation:pre-metered coatings such as patch die coating, slot or extrusioncoating, slide or cascade coating, curtain coating; roll coating such asknife over roll coating, forward and reverse roll coating; gravurecoating; dip coating; spray coating; meniscus coating; spin coating;brush coating; air knife coating; silk screen printing processes;electrostatic printing processes; thermal printing processes; inkjetprinting processes; and other similar techniques.) Thus, the resultingdisplay can be flexible. Further, because the display medium can beprinted (using a variety of methods), the display itself can be madeinexpensively.

However, the electro-optical properties of encapsulated electrophoreticdisplays could still be improved. Among the remaining problems ofencapsulated electrophoretic displays are so-called “rapid white statedegradation”, a relatively rapid decline in the reflectivity of thewhite optical state of the display during the first few days ofoperation. Another problem, which is discussed in detail in theaforementioned 2003/0137521 and WO 03/107315, is so-called “dwell statedependency”, which causes the change in optical state of anelectrophoretic medium produced by a specific waveform to vary dependingupon the time for which the electrophoretic medium has been in aparticular optical state before the waveform is applied. Finally, it isadvisable to make the white optical state of the display whiter and theblack optical state darker.

As discussed in the aforementioned E Ink and MIT patents andapplications, the presently preferred form of electrophoretic mediumcomprises white titania and carbon black particles in a hydrocarbonsuspending fluid, this hydrocarbon being used alone or in admixture witha chlorinated hydrocarbon or other low dielectric constant fluid. Mostother prior art electrophoretic displays which require a black pigmenthave also used carbon black for this purpose, apparently largely becausethe material is readily available in mass quantities and veryinexpensive. However, the present inventors and their co-workers havediscovered that the aforementioned problems with prior artelectrophoretic displays are associated with the use of carbon black forthe black electrophoretic particles. Carbon black has a complex andpoorly understood surface chemistry, which may vary widely with thespecific raw material (typically petroleum) and the exact process usedfor the carbon black production. Carbon black pigment particles alsohave a poorly understood aggregate, fractal structure. Furthermore,carbon black is notoriously effective in adsorbing gases and liquidswith which it comes into contact, and such adsorbed gases and liquidscan change the physicochemical properties of the carbon black surface.Hence, it is difficult to ensure consistent surface properties of carbonblack from batch to batch. This is especially problematic inelectrophoretic displays, since the electrophoretic particles used aretypically so small (of the order of 1 μm) that their properties aredominated by the properties of their surfaces.

It has also been discovered (although this information is not disclosedin the prior art) that carbon black presents certain peculiardifficulties in obtaining proper charging of particles in oppositecharge dual particle electrophoretic displays. Specifically, it has beenfound that when using carbon black and titania as the black and whiteparticles respectively in an opposite charge dual particleelectrophoretic display, combinations of charging agents and othermaterials which produce all positively charged carbon black particlestend to produce a minor proportion of titania particles which are alsopositively charged. The resultant mixture of negatively and positivelycharged titania particles leads to contamination of the extreme opticalstates of the medium, thus adversely affecting its contrast ratio.

Carbon black also has a low density. While this does not affect theoperation of the display itself, it does complicate the manufacture ofencapsulated dual particle displays. For reasons explained in several ofthe aforementioned E Ink and MIT patents, it is desirable that anencapsulated electrophoretic medium comprises a single, substantiallyclose-packed layer of capsules. Also, when such an electrophoreticmedium is produced by coating capsules on to a substrate, it isdesirable that the exposed surface of the capsule layer be reasonablyflat, since otherwise difficulties may be encountered in laminating thecapsule layer to other layers in the final display. Production of such asubstantially close-packed layer with a reasonably flat exposed surfaceis best achieved by coating capsules which are of substantially the samesize. However, the encapsulation processes described in aforementioned EInk and MIT patents produce capsules having a broad range of sizes, andhence it is necessary to separate out the capsules having the desiredrange of sizes. Many useful processes for sorting capsules by size relyupon using the density difference between the capsules and a surroundingmedium to effect the desired sorting. The low density of carbon black,coupled with the small concentration at which it is used in mostelectrophoretic media, lead to capsule densities close to that of water,hindering the sorting process.

There is thus a need for a black particle for use in electrophoreticmedia that does not suffer from the problems associated with the use ofcarbon black. However, the search for such a black particle is subjectto considerable difficulties. Although the optical properties ofnumerous pigments are of course known from their use in the paint andsimilar industries, a pigment for use in an electrophoretic display mustpossess several properties in addition to appropriate opticalproperties. The pigment must be compatible with the numerous othercomponents of the electrophoretic medium, including the suspendingfluid, any other pigment particles present, charge control agents andsurfactants typically present in the suspending fluid, and the capsulewall material (if a capsule wall is present). The pigment particles mustalso be able to sustain a charge when suspended in the suspending fluid,and the zeta potentials of the particles caused by such charges shouldall be of the same polarity and should not extend over an excessivelywide range, or the electrophoretic medium may not have desirableelectro-optic properties; for example, if some particles have very lowzeta potentials, a very long driving pulse may be required to move suchparticles to a desired position within the electrophoretic medium,resulting in slow switching of the medium. It will be appreciated thatsuch information relating to the ability of pigment particles to acquireand hold charges is not available for most pigments potentially usablein an electrophoretic display, since such electrical properties areirrelevant to the normal commercial uses of the pigments.

The aforementioned copending application Ser. No. 09/140,846 mentionsnumerous pigments that are potentially useful in an electrophoreticdisplay. It has now been found that one of these pigment, namely copperchromite (Cu₂Cr₂O₃) has particular advantages for use in electrophoreticmedia and displays, and this invention relates to electrophoretic mediaand displays containing copper chromite. It has also been found thatcertain surface treatments, in particular the formation of layers ofsilica and formation of polymers attached to the copper chromiteparticle, substantially as described in the aforementioned copendingapplications Ser. Nos. 10/063,803 and 60/481,572, improve theperformance of the copper chromite particle in electrophoretic media anddisplays, and this invention also relates to such modified copperchromite particles and electrophoretic media and displays containingthem.

SUMMARY OF INVENTION

Accordingly, in one aspect this invention provides an electrophoreticmedium comprising at least one electrically charged particle suspendedin a suspending fluid and capable of moving through the fluid onapplication of an electrical field to the fluid. According to thepresent invention, the at least one electrically charged particlecomprises copper chromite.

In such an electrophoretic medium, the at least one electrically chargedparticle may have an average diameter of from about 0.25 to about 5 μm.The at least one electrically charged particle may be coated with silicaand/or have a polymer chemically bonded to, or cross-linked around, theat least one particle. It is generally preferred that the polymer bechemically bonded to the at least one particle. The polymer coatingshould correspond to between 5 and 500 mg/m² of particle surface area(as measured by the BET technique or other suitable method), morepreferably to between 10 and 100 mg/m², and most preferably to between20 and 100 mg/m².The polymer may comprise from about 1 to about 15percent by weight, preferably from about 2 to about 8 percent by weight,of the at least one particle. The polymer may comprise charged orchargeable groups, for example amino, carboxyl, or sulfonate groups. Thepolymer may also comprise a main chain and a plurality of side chainsextending from the main chain, each of the side chains comprising atleast about four carbon atoms. The polymer may be formed by radicalpolymerization from acrylate, methacrylate, styryl, or other vinylmonomers or mixtures thereof. The polymerization can take place in onestep or in multiple sequential steps.

As described in more detail below, the polymer may be bonded to theparticle via a residue of a functionalization agent, for example asilane. The residue of the functionalization agent may comprise chargedor chargeable groups.

The electrophoretic medium of the invention may be of any of theaforementioned types but preferably comprises at least one secondparticle having at least one optical characteristic differing from thatof the copper chromite particle(s), the at least one second particlealso having an electrophoretic mobility differing from that of thecopper chromite particle(s). The copper chromite particles and thesecond particles may bear charges of opposite polarity. The secondparticles may be substantially white, a preferred white pigment for thispurpose being titania. In the electrophoretic medium of the invention,the suspending fluid may comprise a hydrocarbon, or a mixture of ahydrocarbon and a halogenated hydrocarbon.

The electrophoretic medium of the present invention may be of theencapsulated type and comprise a capsule wall within which thesuspending fluid and the at least one particle are retained. Such anencapsulated medium may comprise a plurality of capsules each comprisinga capsule wall and the suspending fluid and at least one particleretained therein, the medium further comprising a polymeric bindersurrounding the capsules.

This invention extends to an electrophoretic display comprising anelectrophoretic medium of the present invention and at least oneelectrode disposed adjacent the electrophoretic medium for applying anelectric field to the medium. In such an electrophoretic display, theelectrophoretic medium may comprise a plurality of capsules.Alternatively, the electrophoretic medium may be of thepolymer-dispersed type and comprise a plurality of droplets comprisingthe suspending fluid and a continuous phase of a polymeric materialsurrounding the droplets. The electrophoretic display may also be of themicrocell type and comprise a substrate having a plurality of sealedcavities formed therein, with the suspending fluid and the copperchromite particles retained within the sealed cavities.

In another aspect, this invention provides a copper chromite particlehaving a silica coating. Preferably, such a copper chromite particle hasa mean diameter in the range of about 0.25 to about 5 μm.

In another aspect, this invention provides a copper chromite particlehaving a polymer chemically bonded to, or cross-linked around, theparticle. Preferably, the polymer is chemically bonded to the particle.The polymer coating should correspond to between 5 and 500 mg/m² ofpigment surface area (as measured by the BET technique or other suitablemethod), more preferably to between 10 and 100 mg/m², and mostpreferably to between 20 and 100 mg/m². The polymer may comprise fromabout 1 to about 15 percent by weight, preferably from about 2 to about8 percent by weight, of the particle. The polymer (or a residue of afunctionalization agent bonding the polymer to the particle, aspreviously discussed) may comprise charged or chargeable groups, forexample amino groups. The polymer may comprise a main chain and aplurality of side chains extending from the main chain, each of the sidechains comprising at least about four carbon atoms. The polymer may beformed by radical polymerization from acrylate. methacrylate, styryl, orother vinyl monomers or mixtures thereof

In another aspect, this invention provides a process for producing apolymer-coated copper chromite particle; this process comprises:

(a) reacting the particle with a reagent having a functional groupcapable of reacting with, and bonding to, the particle, and also havinga polymerizable or polymerization-initiating group, thereby causing thefunctional group to react with the particle surface and attach thepolymerizable group thereto; and

(b) reacting the product of step (a) with at least one monomer oroligomer under conditions effective to cause reaction between thepolymerizable or polymerization-initiating group on the particle and theat least one monomer or oligomer, thereby causing the formation ofpolymer bonded to the particle.

In this process, the copper chromite particle may be coated with silicaprior to step (a). The reagent used in step (a) may comprise a silanecoupling group, for example a trialkoxysilane coupling group. Thereagent may further comprise an amino group. For example, the reagentmay comprise aN-[3-(trimethoxysilyl)propyl]-N′-(4-vinylbenzyl)ethylenediamine salt.The monomer or oligomer may comprise at least one acrylate.methacrylate, styryl, or other vinyl monomer or mixtures thereof, forexample lauryl methacrylate, 2-ethylhexyl methacrylate, hexylmethacrylate, and other alkyl esters of methacrylic acid and acrylicacid, hexafluorobutyl methacrylate and other halogen-containingmonomers, styrene and substituted styrene derivatives, vinyl halides,for example, vinyl chloride, vinylidene chloride, vinylidene fluoride,and the like, vinyl ethers and esters, for example, methyl vinyl ether,and vinyl acetate, and other substituted vinyl derivatives, includingmaleic anhydride and maleic esters, vinyl pyrrolidone and the like. Inaddition, the polymerization may include other reagents to modify themolecular weight of the polymer, for example, chain-transfer agents,such as carbon tetrabromide, mercapto-derivatives, or other suchmaterials as known in the vinyl polymerization art.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows schematically a process for preparing electrophoreticparticles by the process of the present invention.

FIG. 2 is a graph comparing the electro-optic properties of dualparticle electrophoretic media using copper chromite and carbon blackelectrophoretic particles, as described in Example 5 below.

FIG. 3 is a graph showing the threshold behavior of a dual particleelectrophoretic medium using a copolymer-coated copper chromite pigment,as described in Example 7 below.

DETAILED DESCRIPTION

As already mentioned, this invention relates to electrophoreticparticles comprising copper chromite, to processes for the production ofsuch particles, and to electrophoretic media and displays containingsuch particles.

Copper chromite has a number of advantages for use as an electrophoreticparticle. It is readily available commercially in substantialquantities, the presently preferred commercial form being the materialsold under the trade name Shepherd Black 1G by Shepherd Color Company,4539 Dues Drive, Cincinnati, Ohio 45246. This pigment is a calcinedparticulate pigment with an average size of about 1 μm, specific gravityof 5.5, and surface area of 2.7 m²/g. A second pigment, Shepherd Black1, from the same supplier, has also been found to give satisfactoryresults; this pigment has the same composition as Black 1G but aslightly larger particle size (1.2 μm) and slightly smaller surface area(2.1 m²/g).

Copper chromite has a more consistent surface chemistry than carbonblack. For optimum performance as an electrophoretic particle, thecopper chromite should be provided with a polymer coating of the typedescribed in the aforementioned copending application Ser. No.10/063,803. In one embodiment of the invention, it is preferred that thecopper chromite be coated with silica prior to the formation of thepolymer coating. The silica coating may also be prepared by a process asdescribed in the aforementioned copending application Ser. No.10/063,803. The polymer coating, and optional silica coating, providethe copper chromite with an engineered and well-defined surface which isaccurately reproducible from batch to batch.

The polymer coating may be of any of the types described in theaforementioned copending applications Ser. Nos. 10/063,803 and60/481,572, and in view of the large number of processes for formingsuch polymer coatings described in these copending applications, onlythe presently preferred processes for forming the polymer coatings oncopper chromite particles will be described in detail below, although itis again stressed that any of the processes for forming such polymercoatings described in these copending applications may be used in thepresent invention. Thus, although the polymer coating may be chemicallybonded to, or cross-linked around, the copper chromite particle, theformer is generally preferred. The polymer coating is preferably formedby (a) reacting the particle with a reagent (a “functionalizationagent”) having a functional group capable of reacting with, and bondingto, the particle, and also having a polymerizable orpolymerization-initiating group, thereby causing the functional group toreact with the particle surface and attach the polymerizable groupthereto; and (b) reacting the product of step (a) with at least onemonomer or oligomer under conditions effective to cause reaction betweenthe polymerizable or polymerization-initiating group on the particle andthe at least one monomer or oligomer, thereby causing the formation ofpolymer bonded to the particle.

The reagent used step (a) of this process has a functional group capableof bonding to the particle and a polymerizable orpolymerization-initiating group, which is used in step (b) to causeformation of polymer. If, as is typically the case, the copper chromiteparticle is coated with silica prior to step (a), the reagent needs toreact with the silica coating rather than a copper chromite surface, andconvenient reagents for reacting with such a silica surface are silanes,especially trialkoxysilanes. The polymerizable group is conveniently avinyl group. The polymer coating desirably contains charged orchargeable groups, since the presence of such groups assists incontrolling and maintaining a stable charge on the copper chromiteparticles in the electrophoretic medium. Such groups are convenientlypresent in the reagent used in step (a), although they can also bepresent in the monomers used in step (b); for example, if a positivelycharged copper chromite particle is desired, the reagent may containamino groups. (In practice, it is generally convenient to make thecopper chromite particles positively charged; the aforementioned E Inkand MIT patents and applications describe opposite charge dual particleelectrophoretic media containing negatively charged white titaniaparticles and positively charged carbon black particles. When replacingthe carbon black particles with copper chromite particles in accordancewith the present invention, it is convenient to keep the same chargingscheme with the negatively charged white titania particles and to makethe black copper chromite particles positively charged.) Preferred aminoreagents for this purpose areN-[3-(trimethoxysilyl)propyl]-N′-(4-vinylbenzyl)ethylenediamine salts,especially the hydrochloride, for convenience hereinafter referred to as“TMSP”. If a negatively charged copper chromite particle is desired,conveniently the silane of the formula:H₂C═C(CH₃)CO₂(CH₂)₃Si(OCH₃)₃sold commercially under the Registered Trade Mark Z6030, is used.

The monomer used in step (b) of the process for forming the polymercoating is conveniently an acrylate or methacrylate, although as alreadymentioned a wide variety of other monomers may be used; as will readilybe apparent to those skilled in polymer synthesis, it is normallynecessary to include a free radical polymer initiator in the reactionmixture to enable the vinyl or other polymerizable group derived fromthe reagent used in step (a) of the process to react with the acrylateor methacrylate monomer to form the polymer. As discussed in more detailin the aforementioned copending application Ser. No. 10/063,803, it isgenerally desirable that the polymer used to form the polymer coatingcomprise a main chain and a plurality of side chains extending from themain chain, each of the side chains comprising at least about fourcarbon atoms; this type of “brush” polymer is believed to spread outinto a brush or tree-like structure in the hydrocarbon suspending fluidscommonly used in electrophoretic media, thus increasing the affinity ofthe electrophoretic particles for the suspending fluid and thus thestability of the particle dispersion. Side chains substantially largerthan four carbon atoms are useful in achieving an optimum brushstructure, and the presently preferred monomers for forming the polymercoating are lauryl methacrylate and 2-ethylhexyl methacrylate. Forreasons discussed in detail in the aforementioned copending applicationSer. No. 60/481,572, it may be advantageous to include a minorproportion of a second monomer, for example styrene or a fluorinatedmonomer, with the lauryl- or 2-ethylhexyl methacrylate or other acrylateor methacrylate monomer.

The electrophoretic properties of the copper chromite particle may beaffected by the amount of polymer coating. Generally, it is preferredthat the polymer comprise between about 5 and about 500 mg/m² of pigmentsurface area (as measured by the BET technique or other suitablemethod), more preferably to between 10 and 100 mg/m², and mostpreferably to between 20 and 100 mg/m². As illustrated in the Examples,below, one preferred process produces a polymer coating constitutingabout 4 percent by weight of the particle, corresponding to about 40 mgm⁻² using the aforementioned Shepherd 1G as the base particle.

For various reasons discussed below, in step (b) of the process, it isoften desirable to incorporate more than one type of monomer (oroligomer) into the polymer coating, and two different approaches may beused to form such “multiple monomer” coatings. The first approach is asingle stage copolymerization using a mixture of the relevant monomers.The second approach uses two or more successive polymerization steps (ineffect, splitting step (b) of the process into two or more sub-steps),each of which uses a different monomer or mixture of monomers. Forexample, as illustrated in the Examples below, a first polymerizationwith lauryl methacrylate may be followed by a second polymerization withstyrene. It will readily be apparent to those skilled in polymertechnology that the properties of the polymer-coated particles producedby the two approaches may differ, since the first approach produces arandom copolymer, whereas the second approach produces a block copolymeror mixture of homopolymers.

The electrophoretic media and displays produced using the copperchromite particles of the present invention may be of any of theaforementioned types. Thus, the media and displays may be of the singleparticle, opposite polarity dual particle or same polarity dual particletypes. Similarly, the media and displays may be of the unencapsulated,encapsulated, polymer-dispersed and microcell types. The replacement ofthe carbon black particles typically used in prior art electrophoreticmedia and displays with copper chromite particles in accordance with thepresent invention will typically require the use of a substantiallygreater weight of copper chromite particles than carbon black particlesbecause of the much greater density of copper chromite; the optimumamount of copper chromite to be used in any specific electrophoreticmedium may be determined empirically, and the Examples below illustratetypical amounts. Apart from this difference in amount of pigment,electrophoretic media and displays using copper chromite can generallyuse the same technology as similar media and displays using carbonblack, and the reader is referred to the aforementioned E Ink, MIT andother patents and applications for fuller information.

FIG. 1 of the accompanying drawings shows schematically a process forpreparing electrophoretic particles in accordance with the presentinvention, this process being essentially similar to that described inthe aforementioned copending application Ser. No. 10/063,803. In thefirst stage of the process, a copper chromite pigment particle 100 iscoated with silica to form a silica coating 102; this step is fullydescribed in the aforementioned copending application Ser. No.10/063,803 and a preferred process is described in Example 1 below.Next, the silica-coated pigment is treated with a bifunctional reagenthaving one functional group which reacts with the silica surface, and asecond, charge control group, thus producing a surface functionalizedcopper chromite pigment bearing charge control groups 104 on itssurface. The bifunctional reagent also provides a site for the formationof polymer on the pigment particle. Finally, as shown in FIG. 1, thesurface functionalized pigment is contacted with one or more monomers oroligomers under conditions effective to cause formation of polymer 106attached to the charge control groups, thereby producing a colloidallystable copper chromite pigment ready for use in an electrophoreticmedium.

The following Examples, are now given, though by way of illustrationonly, to show preferred reagents, conditions and techniques useful inthe practice of the present invention.

EXAMPLE 1 Silica Coating of Copper Chromite Pigment

A dispersion was prepared consisting of Shepherd Black 1G (50 g), water(420 mL) and sodium silicate solution (6 mL of a solution containingapproximately 14% of sodium hydroxide and 27% of silica, available fromAldrich Chemical Company); the dispersion was sonicated for one hour.The dispersion was then added to a 1 L three-necked flask equipped witha stirring bar, reflux condenser (open to the air) and two additionfunnels equipped with needle-valve control stopcocks. The flask waspartially immersed in a silicone oil bath and heated to 100° C. withrapid stirring over a period of one hour. During this heating period,the first addition funnel was charged with 0.22M sulfuric acid (150 mL)and the second with a mixture of the aforementioned sodium silicatesolution (11 mL) and water (70 mL). After the silicone oil bath reached100° C., the two solutions in the addition funnels were simultaneouslyadded to the dispersion in the flask over a period of two hours. Theheat was then turned off and the bath and flask were allowed to coolslowly to room temperature. Stirring was then continued overnight. Thesilicated dispersion was then poured into centrifuge bottles andcentrifuged at 3600 rpm for 10 minutes. The supernatant solution wasdecanted and discarded, and fresh deionized water was added to thebottles, which were shaken to re-suspend the sediment, and thenre-centrifuged at 3600 rpm for 10 minutes. After decantation of thesupernatant solution a second time, the bottles were covered loosely andthe sedimented pigment allowed to air dry. The pigment thus produced wasthen placed in a crystallization dish and dried in a 105° C. oven fortwo days. The dried pigment was hand ground using a mortar and pestleand sieved using a 1.0 mm stainless steel mesh screen. Finally, thesieved pigment was finely ground using 2 inch (51 mm) jet millMicronizer (available from Sturtevant Inc., Hanover, Mass.) using 100psi (0.8 mPa) air and a pigment feed rate of about 250 g/hr.

EXAMPLE 2 Silane Treatment of Silicated Copper Chromate Pigment

The silica-coated copper chromite pigment produced in Example 1 was usedwithout further treatment in a silanization process. For this purpose, amixture of 300 ml of ethanol, 30 ml of water and 40 g of a 40 weightpercent solution ofN-[3-(trimethoxysilyl)propyl]-N′-(4-vinylbenzyl)ethylenediaminehydrochloride (available from United Chemical Technologies, 2731 BartramRoad, Bristol, Pa. 19007-6893) in methanol was stirred rapidly for 7minutes, the pigment was added thereto, and the resultant mixture wasstirred for a further 5 minutes. The resultant suspension was pouredinto plastic bottles and centrifuged at 3500 rpm for 30 minutes. Thesupernatant liquor was decanted, and the silanized pigment redispersedin ethanol and centrifuged at 3500 rpm for 30 minutes, and the liquiddecanted. The washing was repeated, and the pigment finally dried in airfor 18 hours, then under vacuum at 70° C. for 2 hours. The amount ofsurface functionalization was estimated by thermogravimetric analysis(TGA), which indicated the presence of 1.5–1.7% of volatile (organic)material, implying a surface coverage of 19–22 μmol/m², representingsubstantially more than a monolayer of the silane.

EXAMPLE 3 Polymer Coating of Silanized Copper Chromate Pigment

The silanized pigment produced in Example 2 (50 g) was placed in around-bottomed flask with toluene (50 g) and 2-ethylhexyl methacrylatemonomer (50 g). The resultant mixture was stirred rapidly under anitrogen atmosphere (argon may alternatively be used) for 20 minutes,then slowly heated to 50° C. and AIBN (0.5 g in 10 ml of toluene) addedquickly. The suspension was then heated to 65° C. and stirred at thistemperature under nitrogen for a further 18 hours. The resultantsuspension was poured into plastic bottles, the flask being washed outwith ethyl acetate to remove residual product and the ethyl acetatesolution added to the bottles. The bottles were centrifuged at 3500 rpmfor 30 minutes. The supernatant liquor was decanted, and thepolymer-coated pigment re-dispersed in ethyl acetate and centrifuged at3500 rpm for 30 minutes, and the liquid decanted. The washing wasrepeated, and the pigment dried in air until a workable powder wasobtained, and then under vacuum at 65° C. for 6 to 18 hours. Thispolymerization resulted in a final pigment containing 3.5–4.5% volatilematerial by TGA.

EXAMPLE 4 Preparation of Dual Particle Electrophoretic DisplayContaining Copper Chromite Pigment

This Example illustrates the conversion of the polymer-coated copperchromite pigment produced in Example 3 above to an encapsulated oppositecharge dual particle electrophoretic display using a processsubstantially as described in the aforementioned copending applicationSer. No. 10/065,803. The titania pigment used was also polymer-coatedessentially as described in Example 28 of this copending application.

Part A: Preparation of Internal Phase

An internal phase (i.e., an electrophoretic suspension comprising thetwo types of electrophoretic particles, the suspending fluid and certainadditives) was prepared by combining the following constituents in a 4Lplastic bottle:

Copper chromite (53.4 weight percent dispersion in Isopar G) 1070.09 g

Titania (60 weight percent dispersion in Isopar G): 2380.95 g

Polyisobutylene (13 weight percent dispersion in Isopar G) 143.08 g

Solsperse 17K (10 weight percent solution in Isopar G) 400.00 g

Additional Isopar G: 5.88 g

The resultant mixture was sonicated for 1 hour by immersion in asonicating water bath, and then rolled overnight on a mechanical rollerto produce an internal phase ready for encapsulation.

Part B: Encapsulation

Deionized water (2622 g) was added to a 4 L jacketed reactor equippedwith a prop stirrer and warmed to 42.5° C. using a circulatingcontrolled temperature bath. Gelatin (66.7 g) was hydrated by sprinklingthe powdered material onto the surface of the water in the reactor andallowing the suspension to stand for 1 hour. The stirrer was then set at100 rpm and the solution stirred for 30 minutes. The stirring rate wasthen increased to 350 rpm and the internal phase prepared in Part Aabove was added over a period of five minutes using a sub-surfaceaddition funnel. Immediately after the addition of the internal phasehad been completed, the stirring rate was increased to 500 rpm for 1hour to produce internal phase droplets in the correct size range.During this emulsification, a solution of 66.7 g of gum acacia dissolvedin 656 g of water was warmed to 40° C. At the end of the emulsificationperiod, the acacia solution was added all at once to the internal phasesuspension, and the pH of the suspension was adjusted to 5.0 by additionof 3.5 g of 10 weight percent acetic acid.

The bath temperature on the circulator was then changed to 8° C. Whenthe temperature of the suspension reached 10° C. (after 2–3 hours), 16.7g of 50 percent glutaraldehyde solution in water was added. The bathtemperature was then set at 25° C., and the suspension warmed andstirred overnight. The resultant encapsulated material was isolated bysedimentation, washed with deionized water, and size-separated bysieving, using sieves of 38 and 25 μm mesh. Analysis using a CoulterMultisizer showed that the resulting capsules had a mean size of 40 μmand more than 85 percent of the total capsule volume was in capsuleshaving the desired size of between 30 and 50 μm.

Part C: Preparation of Capsule Slurry

Following the size separation described in Part B above, the capsuleswere allowed to settle and all excess water was decanted. The resultingcapsule slurry was adjusted to pH 8 with 1 weight percent ammoniumhydroxide solution. The capsules were then concentrated bycentrifugation and the supernatant liquid discarded. The capsules weremixed with an aqueous urethane binder at a ratio of 1 part by weightbinder to 8 parts by weight of capsules and 0.1 weight percent TritonX-100 surfactant and 0.2 weight percent of hydroxypropylmethyl cellulose(both based on the total weight of capsule slurry and binder) were addedand mixed thoroughly.

Part D: Manufacture of Display

The mixture produced in Part C above was bar-coated, using a 4 mil (101μm) coating slot, on to a 125 μm thick indium-tin oxide (ITO)-coatedpolyester film, the capsules being deposited on the ITO-coated surfaceof the film. The resulting coated film was dried in an oven at 60° C.for 1 hour.

Separately, a polyurethane lamination adhesive was coated on the arelease sheet to form a dried adhesive layer 55 μm thick, and theresultant coated sheet was cut to a size slightly larger than that ofthe capsule-coated film. The two sheets were then laminated together(with the lamination adhesive in contact with the capsule layer) byrunning them through a Western Magnum roll laminator with the top rollset at 279° C. and the bottom roll set at 184° C. to form a front planelaminate as described in copending application Ser. No. 10/249,957,filed May 22, 2003. The front plane laminate was then cut to the desiredsize, the release sheet removed, and the lamination adhesive layerthereof laminated to a backplane comprising a polymeric film coveredwith a graphite layer, the lamination adhesive being contacted with thegraphite layer. This second lamination was effected using the samelaminator but with a top roll temperature of 259° C. and a bottom rolltemperature of 184° C. The laminated pixels were cut out using a lasercutter, and electrical connections applied to produce experimentalsingle-pixel displays suitable for use in the electro-optic testsdescribed in Example 5 below.

To provide controls for use in these electro-optic tests, a similarprior art dual particle electrophoretic display was produced using asthe black pigment a polymer-coated carbon black prepared essentially asdescribed in Example 27 of the aforementioned copending application Ser.No. 10/063,803.

EXAMPLE 5 Electro-Optic Properties of Display Containing Copper ChromitePigment

The experimental displays produced in Example 4 above were tested byswitching them through several transitions between their extreme blackand white optical states. To test the variation of white state withapplied pulse length, a display was driven to its black state, then 15 Vsquare wave pulses of varying duration were applied and the resultantwhite state measured. The variation of dark state with applied pulselength was tested in the same way but starting with the display in itswhite state. The reflectance of the state produced by the driving pulsewas measured, and expressed in L* units, where L* has the usual CIEdefinition. FIG. 2 of the accompanying drawings shows the resultsobtained for both the displays of the present invention and the priorart displays.

From FIG. 2 it will be seen that the display of the present inventionhas both whiter white states and darker dark states than the prior artdisplay when operated at the same pulse length and voltage. As a result,the contrast ratio for the display of the present invention issubstantially higher; for example, at a pulse length of 550 ms, theprior art display has a contrast ratio of 8.8, whereas the display ofthe invention has a contrast ratio of 13.7.

Further tests were conducted to measure the dwell state dependence ofthe two types of display. As explained above, dwell state dependence(“DSD”) is a phenomenon which causes the response of a pixel of adisplay to an applied driving pulse to vary with the time for which thepixel has previously remained in the same optical state. This is aproblem in electrophoretic displays, where the time for which a specificpixel remains in the same optical state before being switched to a newoptical state can vary greatly. To test dwell state dependence, theprevious tests were substantially repeated, except that the display wasleft for a rest length (“RL”) of either 50 or 4050 milliseconds in itsprevious state before the driving pulse was applied, and the drivingpulse had a fixed duration of 350 msec at 15 V. The results are shown inthe Table 1 below.

TABLE 1 White White Dark Dark state state state state 50 ms 4050 ms 50ms 4050 ms Display RL RL ΔWS RL RL ΔDS Control 68.7 66.4 −2.3 22.9 26.3+4.3 Present 67.1 65.7 −1.4 24.4 22.4 −2.0 Invention

As is well known to those skilled in display technology, a change lessthan or equal to 2 units in L* is regarded as a satisfactoryperformance, since changes of this magnitude are either invisible to theeye or nearly so, whereas greater changes than this cause visuallyunattractive artifacts. Hence Table 1 shows that the prior art displaywould produce such artifacts under the test conditions, whereas thedisplay of the present invention would not.

The density of the capsules produced in Example 4, Part B above was alsomeasured. It was found that the capsules of the present invention had aspecific gravity of approximately 1.15, whereas the control capsules hada specific gravity of approximately 1.06. Since many techniques for sizeseparation of capsules, such as centrifugation, depend upon thedifference between the density of capsules and water, it will readily beapparent that the capsules of the present invention could be sizeseparated much more quickly than the control capsules, or, from apractical perspective, that a given separation apparatus can process amuch greater throughput of the capsules of the present invention thanthe control capsules, and hence that for any given production rate, thesize and cost of the separation apparatus can be reduced.

EXAMPLE 6 Alternative Process for Silica-Coating and Silanization ofCopper Chromite Pigment

Copper chromite (Shepherd Black 1G, 50 g) was placed in a sodiumsilicate solution (430 ml of a 0.073M solution with 1.9% sodiumhydroxide), and the resultant mixture was rapidly stirred and thensonicated at 30–35° C. The suspension was then heated to 90–95° C. overa period of 1 hour and sulfuric acid (150 mL of a 0.22M solution) andadditional sodium silicate (75 mL of a 0.83M solution with 0.2% sodiumhydroxide) were added simultaneously over a period of 2.5 to 3 hours,with stirring. After these additions had been completed, the reactionmixture was stirred for an additional 15 minutes, then cooled to roomtemperature. Additional sulfuric acid (18 mL of 1M acid) was then addedto the reaction mixture to lower its pH from about 9.5–10 to about 3.The reaction mixture was then placed in plastic bottles and centrifugedat 3700 rpm for 15 minutes, and the supernatant liquid decanted.Immediately after this decantation, deionized water (5 mL) and ethanol(50 mL) were added to each bottle, which was then shaken vigorously. Thebottles were then sonicated for 1 hour. Microscopic investigation of theresultant dispersion revealed well-dispersed primary pigment particles.

The dispersion of silica-coated copper chromite pigment thus producedwas used without any further treatment in a silanization process. Forthis purpose, a mixture of 300 ml of ethanol, 30 ml of water and 40 g ofa 40 weight percent solution ofN-[3-(trimethoxysilyl)propyl]-N′-(4-vinylbenzyl)ethylenediaminehydrochloride in methanol was stirred rapidly for 7 minutes, the pigmentdispersion was added thereto, and the resultant mixture was stirred fora further 5 minutes. The resultant suspension was poured into plasticbottles and centrifuged at 3500 rpm for 30 minutes. The supernatantliquor was decanted, and the silanized pigment redispersed in ethanoland centrifuged at 3500 rpm for 30 minutes, and the liquid decanted. Thewashing was repeated, and the pigment finally dried in air for 18 hours,then under vacuum at 70° C. for 2 hours.

EXAMPLE 7 Copper Chromite Pigment Polymer-Coated with a Mixture ofMonomers

This Example illustrates the use of copper chromite pigments having apolymer coating derived from more than one monomer. These experimentsuse a white titania pigment based on Dupont Ti-Pure R960 and a blackcopper chromite pigment based on Shepherd Black 1G. These particles weresurface functionalized as per FIG. 1 using Z6030 for the white pigmentand TMSP for the black. As previously mentioned, the white particles arenegatively charged when incorporated into an Isopar G suspending fluidincluding Solsperse 17K and Span as charging agents, and the blackparticles are positively charged. The particles with lauryl methacrylatehomopolymer shells are similar to those used in Example 5 above and arereferred to by the letters J and D for white particles and blackparticles respectively. Modification of the polymer shell will bedescribed as follows: for particles made using a random copolymer,one-stage polymerization, the comonomer will be identified by the lettercodes indicated in Table 2 in parentheses after the appropriate pigmentindicator, together with a number indicating the mole fraction of thecomonomer used in the polymerization reaction. The majority comonomer ineach case is lauryl methacrylate. Thus, the notation J(BMA15) indicatesa white pigment made from a polymerization mixture comprising 15 mole %t-butyl methacrylate and 85 mole % lauryl methacrylate. A two-stagepolymerization will be indicated by a plus sign; in general, the moleratio of polymers for these particles is not so well known, so that noindication is given about composition. Thus, J(+St) indicates a whitepigment made by a two-stage polymerization. The starting material wasthe pigment with 100 mole percent lauryl methacrylate used in the firststage polymerization; the second stage polymerization was carried outusing only styrene as the polymerizable monomer.

TABLE 2 Monomer Shorthand symbol styrene St t-butyl methacrylate TBMAvinyl pyrrolidone VP

The procedures used for formation of the polymer shells were as follows:

1. Standard Lauryl Methacrylate Polymerization

A single-neck, 250 mL round bottom flask was equipped with a magneticstir bar, reflux condenser and an argon/or nitrogen inlet and placed ina silicon oil bath. To the flask was added 60 g of silane coated pigmentwhich was pulverized into a fine powder using a mortar and pestle,followed by 60 mL of lauryl methacrylate (LMA, Aldrich) and 60 mL oftoluene. The reaction mixture was then stirred rapidly and the flaskpurged with argon or nitrogen for one hour. During this time the siliconbath was heated to 50° C. During the purge, 0.6 g of AIBN(2,2′-azobisisobutyronitrile, Aldrich) was dissolved in 13 mL oftoluene. At the end of the one-hour purge the AIBN/toluene solution wasadded quickly with a glass pipette. The reaction vessel heated to 65° C.and allowed to stir overnight. At the end of the polymerization, to theviscous reaction mixture was added 100 mL of ethyl acetate and themixture allowed to stir for another 10 minutes. The mixture was pouredinto plastic bottles, centrifuged for 15–20 minutes at 3600 rpm anddecanted. Fresh ethyl acetate was added to centrifuged pigment, whichwas resuspended by stirring with a stainless steel spatula andsonicating for 10 minutes. The pigment was washed twice more with ethylacetate followed above procedure. The pigment was allowed to air dryovernight followed by drying under high vacuum for 24 hours. The freepolymer in bulk solution was precipitated in methanol and dried undervacuum. The molecular weight of free polymer in the solution wasdetermined by gas phase chromatography (GPC). The polymer bound on thepigment was measured by TGA.

2. Co-Polymerization of Z6030-Coated Titania

A single-neck, 250 mL round bottom flask was equipped with a magneticstir bar, reflux condenser and an argon/or nitrogen inlet and placed ina silicon oil bath. To the flask were added 60 mL of toluene, 60 g ofPD65 (Z6030-coated titania (derived from R960 Titanium Oxide pigment,supplied by Dupont de Nemours and Company, Inc.), lauryl methacrylate(LMA) and a second monomer, such as styrene, t-butyl methacrylate,1-vinyl-2-pyrrolidinone, hexafluorobutyl acrylate or methacrylate,N-isopropylacrylamide or acrylonitrile (Aldrich), the amounts of LMA andsecond monomer depending on the desired monomer ratio. The ratios ofLMA/second monomer were usually 95/5, 85/15 and 75/25. The reactionmixture was then stirred rapidly and the flask purged with argon ornitrogen for one hour. During this time the silicon bath was heated to50° C. During the purge, 0.6 g of AIBN was dissolved or partiallydissolved in 13 mL of toluene. At the end of the one hour purge theAIBN/toluene solution was added quickly with a glass pipette. Thereaction vessel was heated to 65° C. and allowed to stir overnight. Tothe viscous reaction mixture was added 100 mL of ethyl acetate and theresultant mixture was allowed to stir for another 10 minutes. Themixture was poured into plastic bottles and centrifuged for 15–20minutes at 3600 rpm and decanted. Fresh ethyl acetate was added to thecentrifuged pigment, and the resulting mixture stirred with a stainlesssteel spatula and sonicated for 10 minutes. The pigment was washed twicemore with ethyl acetate, centrifuged and decanted. The pigment wasallowed to air dry overnight followed by drying under high vacuum for 24hours. The free polymer in bulk solution was precipitated in methanoland dried under vacuum. The molecular weight of free polymer in thesolution was determined by GPC. The polymer bound on the pigment wasmeasured by TGA.

2.1 LMA and 1-vinyl-2-pyrrolidinone Copolymerization of Z6030 CoatedR960

A single-neck, 250 mL round bottom flask was equipped with a magneticstir bar, reflux condenser and an argon/or nitrogen inlet and placed ina silicon oil bath. To the flask were added 60 mL of toluene, 60 g ofPD65 (Z6030 coated Dupont R960), 51 mL of lauryl methacrylate (LMA) and3.3 mL of 1-vinyl-2-pyrrolidinone, corresponding to a mole ratio ofLMA/1-vinyl-2-pyrrolidinone of 85/15. The reaction mixture was thenstirred rapidly and the flask purged with argon or nitrogen for onehour. During this time the silicon bath was heated to 50° C. During thepurge, 0.6 g of AIBN was dissolved or partially dissolved in 13 mL oftoluene. At the end of the one hour purge the AIBN/toluene solution wasadded quickly with a glass pipette. The reaction vessel was sealed,heated to 65° C. and allowed to stir overnight. To the viscous reactionmixture was added 100 mL of ethyl acetate and the resultant mixture wasallowed to stir for another 10 minutes. The mixture was poured intoplastic bottles and centrifuged for 15–20 minutes at 3600 rpm anddecanted. Fresh ethyl acetate was added to the centrifuged pigment, andthe resultant mixture was stirred with a stainless steel spatula andsonicated for 10 minutes. The pigment was washed twice more with ethylacetate, centrifuged and decanted. The pigment was allowed to air dryovernight followed by drying under high vacuum for 24 hours. The freepolymer in bulk solution was precipitated in methanol and dried undervacuum. The molecular weight of free polymer in the solution wasdetermined by GPC. The polymer bound on the pigment was measured by TGA.

3. Second Stage Polymerization from LMA Coated White Pigment

A single-neck, 250 mL round bottom flask was equipped with a magneticstir bar, reflux condenser and an argon/or nitrogen inlet and placed ina silicon oil bath. To the flask was added 60 g of Z6030-coated pigmentthat had been pulverized into a fine powder using a mortar and pestle,followed by 60 mL of lauryl methacrylate and 60 mL of toluene. Thereaction mixture was then stirred rapidly and the flask purged withargon or nitrogen for one hour. During this time the silicon bath washeated to 50° C. During the purge, 0.6 g of AIBN was dissolved in 13 mLof toluene. At the end of the one hour purge the AIBN/toluene solutionwas added quickly with a glass pipette. The reaction vessel was sealed,heated to 65° C. and allowed to stir overnight. At the end of thepolymerization, to the viscous reaction mixture was added 100 mL ofethyl acetate and the resultant mixture was allowed to stir for another10 minutes. The mixture was poured into plastic bottles and centrifugedfor 15–20 minutes at 3600 rpm and decanted. Fresh ethyl acetate wasadded to the centrifuged pigment, and the resultant mixture stirred witha stainless steel spatula. The pigment was washed twice more with ethylacetate following above procedure. The pigment was allowed to air dryovernight followed by drying under high vacuum for 24 hours. The freepolymer in bulk solution was precipitated in methanol and dried undervacuum for GPC test. The free polymer in bulk solution was precipitatedin methanol and dried under vacuum. The molecular weight of free polymerin the solution was determined by GPC. The polymer bound on the pigmentwas measured by TGA.

Another single-neck, 250 mL round bottom flask was equipped with amagnetic stir bar, reflux condenser and an argon/or nitrogen inlet andplaced in a silicon oil bath. To the flask was added 50 g of theLMA-coated pigment prepared above, which was pulverized into a finepowder using a mortar and pestle, followed by 85 mL of toluene and 16 gof styrene. The reaction mixture was then stirred rapidly and the flaskpurged with argon or nitrogen for one hour. During this time the siliconbath was heated to 50° C. During the purge, 0.4 g of AIBN was dissolvedin 10 mL of toluene. At the end of the one hour purge the AIBN/toluenesolution was added quickly with a glass pipette. The reaction vessel wassealed, heated to 65° C. and allowed to stir overnight. At the end ofpolymerization, to the viscous reaction mixture were added 100 mL ofethyl acetate and the resultant mixture was allowed to stir for another10 minutes. The mixture was poured into plastic bottles and centrifugedfor 15–20 minutes at 3600 rpm and decanted. Fresh ethyl acetate wasadded to the centrifuged pigment, and the resultant mixture was stirredwith a stainless steel spatula and sonicated for 10 minutes. The pigmentwas washed twice more with ethyl acetate following the above procedure.The pigment was allowed to air dry overnight followed by drying underhigh vacuum for 24 hours. The free polymer in bulk solution wasprecipitated in methanol and dried under vacuum. The molecular weight offree polymer in the solution was determined by GPC. The polymer bound onthe pigment was measured by TGA.

4. Co-Polymerization of TMSP-Coated Shepherd Black 1G

A single-neck, 250 mL round bottom flask was equipped with a magneticstir bar, reflux condenser and an argon inlet and placed in a siliconoil bath. To the flask was added 60 g of TMSP-coated copper chromitepigment, which was pulverized into a fine powder using a mortar andpestle, followed by 1.2 mL of styrene, 57 mL of lauryl methacrylate and60 mL of toluene. The reaction mixture was then stirred rapidly and theflask purged with argon or nitrogen for one hour and the silicon bathwas heated to 50° C. During the purge, 0.6 g of AIBN was dissolved orpartially dissolved in 13 mL of toluene. At the end of the one-hourpurge the AIBN/toluene solution was added quickly with a glass pipette.The reaction vessel was sealed, heated to 65° C. and allowed to stirovernight. To the viscous reaction mixture was added 100 mL of ethylacetate and the resultant mixture was allowed to stir for another 10minutes. The mixture was poured into plastic bottles and centrifuged for15–20 minutes at 3600 rpm and decanted. Fresh ethyl acetate was added tothe centrifuged pigment, and the resultant mixture was stirred with astainless steel spatula and sonicated for 10 minutes. The pigment waswashed twice more with ethyl acetate, centrifuged and decanted. Thepigment was allowed to air dry overnight followed by drying under highvacuum for 24 hours. The free polymer in bulk solution was precipitatedin methanol and dried under vacuum. The molecular weight of free polymerin the solution was determined by GPC. The polymer bound on the pigmentwas measured by TGA.

Preparation of Internal Phase Containing LMA-Coated ElectrophoreticParticles

An internal phase was formulated from (a) 85 g of a stock solution ofthe J pigment containing 60% by weight of LMA-coated titania in IsoparG; (b) 42.5 g of a stock solution of the D pigment containing 60% byweight of LMA-coated copper chromite in Isopar G; (c) 10.71 g of a stocksolution containing 10% by weight of Solsperse 17000 in Isopar G; (d)31.03 g of Isopar G; and (e) 0.77 g Span 85 (a non-ionic surfactant).

To a 250 mL plastic bottle were added the J and D stock solutions,followed by the addition of the Solsperse 17000 solution and the Span 85and finally the remaining solvent. The resultant internal phase wasshaken vigorously for approximately 5 minutes and then placed on a rollmill overnight (at least 12 hours).

Preparation of Internal Phase using Copolymer-Coated White Pigment andLMA-Coated Black Pigment

An internal phase was formulated from (a) 40 g of a stock solution of amodified J pigment containing 60% by weight of an LMA/TBMA-coatedtitania (mole ratio 85/15) in Isopar G; (b) 20 g of a stock solution ofthe D pigment containing 60% by weight of LMA-coated copper chromite inIsopar G; (c) 5.04 g of a stock solution containing 10% by weight ofSolsperse 17000 in Isopar G; (d) 14.60 g of Isopar G; and (e) 0.36 gSpan 85 (a non-ionic surfactant).

This internal phase was mixed and stored in the same way as the previousinternal phase described above.

Preparation of Internal Phase using LMA-Coated White Pigment andCopolymer-Coated Black Pigment

An internal phase was formulated from (a) 40 g of a stock solution ofthe same j pigment as in the first internal phase, this stock solutioncontaining 60% by weight of an LMA-coated titania in Isopar G; (b) 20 gof a stock solution of the D pigment containing 60% by weight ofLMA/St-coated copper chromite (85/15 or 95/5 monomer ratio) in Isopar G;(c) 5.04 g of a stock solution containing 10% by weight of Solsperse17000 in Isopar G; (d) 14.60 g of Isopar G; and (e) 0.36 g Span 85 (anon-ionic surfactant).

This internal phase was mixed and stored in the same way as the previousinternal phases described above.

The internal phases thus produced were encapsulated separately ingelatin/acacia microcapsules substantially as described in Example 4above. The resulting microcapsules were separated by size and capsuleshaving an average particle size of about 35 μm were used in thefollowing experiments. The microcapsules were mixed into a slurry with apolyurethane binder and coated by a roll-to-roll process at a drycoating weight of 18 g m⁻² on to the surface of a 7 mil (177 μm)poly(ethylene terephthalate) (PET) film carrying an ITO layer on onesurface, the microcapsules being deposited on the ITO-covered surface,substantially as described in Example 4 above. The capsule-bearing filmwas then formed into a front plane laminate by laminating it to a layerof a polyurethane lamination adhesive carried on a release sheet, thislamination being effected at 65 psig (0.51 mPa) at a speed of 6inches/min (2.5 mm/sec) using a Western Magnum twin roll Laminator withboth rolls held at 120° C. To provide experimental single-pixel displayssuitable for use in these experiments, pieces of the resultant frontplane laminate has their release sheets removed and were then laminatedat 75° C. to a 5 cm by 5 cm PET film covered with a carbon black layer,which served as the rear electrode of the single pixel display.

Image Stability Measurements

The single pixel displays thus produced were switched using a 500 mssquare wave pulse at 10 V of alternating sign applied to the top plane(ITO layer) relative to the grounded rear electrode. A rest length of 2seconds between pulses was used for shakeup switches. Image stabilitywas measured by switching the pixel to the appropriate optical state(white or black), grounding the top plane, and measuring the opticalreflectivity continuously for 10 minutes. Optical kickback resultingfrom remnant voltages in the binder and lamination adhesive layers wasassumed to decay in 5–10 seconds. The difference in optical state(measured in L* units) between 5 seconds and 10 minutes was taken to bea measure of the image stability. The results shown in Table 3 wereobtained on selected pixels.

TABLE 3 White state IS Black state IS Pigments Sample No. (dL*) (dL*)J/D 1 −6.3 2.7 J(TBMA5)/D 2 −2.2 2.1 J(TBMA15)/D 3 −4.2 1.1 J/D(St5) 4−4.7 2.0 J/D(St15) 5 −4.7 1.6 J(TBMA5)/D(St15) 6 −3.3 1.7J(TMBA15)/D(St5) 7 −1.6 3.0 J(+St)/D 8 −2.2 0.3

The J/D pixel containing the two LMA-coated pigments shows relativelygood black state stability, but less good white state stability (similardisplays using carbon black instead of copper chromite as the blackpigment in the suspending fluid show much worse state stability in bothwhite and dark states, as shown above). Any of the copolymer-coatedelectrophoretic pigments results in an improvement in the imagestability relative to the J/D sample, either in white state or blackstate or in both. Only in one case is the image stability of a“copolymer-coated” pixel less good than the J/D sample(J(TBMA15)/D(St5)), and here the difference is probably withinexperimental pixel-to-pixel variation. The best overall image stabilityis afforded by the display made with white pigment synthesized using thetwo-stage method, with styrene as the second monomer. This display hasgood image stability in the white state, and excellent image stabilityin the black state.

Response Time

The response times of the electrophoretic displays shown in Table 3 weremeasured by measuring the electro-optic response as a function of pulselength at an operating voltage of 10 V. A rest length of 2000 ms wasused for all measurements. The electro-optic response, as measured bythe difference in L* between the white state and the dark state, wasfound to saturate at a certain pulse length, and then decline slightlyat longer pulse lengths. The J/D pixel (Sample No. 1 in Table 3) andsimilar pixels containing 0.3 and 0.9% by weight high molecular weightpoly-isobutylene (PIB) were included as examples of displays having goodimage stability. Table 4 shows the pulse length at which theelectro-optic response achieves 90% of its value with a 1 second pulselength, for the displays of Table 3.

TABLE 4 Time to 90% Pigments Sample No. saturation (ms) J/D (no PIB) 1294 J(TBMA5)/D 2 173 J(TBMA15)/D 3 281 J/D(St5) 4 325 J/D(St15) 5 375J(+St)/D 8 380 J/D + 0.9% PIB 9 740 J/D + 0.3% PIB 10 575

The samples containing PIB had response times that were substantiallylonger than those of samples containing copolymer-coated pigments. Atleast 0.3% PIB is required to obtain adequate image stability in thissystem, and 0.9% PIB is preferred. Thus the use of copolymer-coatedpigments achieves good image stability with response times between 1.5and 4 times faster than the use of PIB.

Thresholds

The pixel described above using white pigment with 5 mole % TBMA andblack pigment with 15 mole % styrene in their polymer shells, inaddition to relatively good image stability, also displayed a largethreshold for operation. The threshold is displayed in FIG. 3, whichshows the dynamic range of the pixel as a function of the appliedvoltage for a number of pulse lengths. The dynamic range is very small(of the order of 1 L*) for applied voltages less than 4–5 volts,independent of the pulse length. but a good dynamic range (greater than30 L*) is exhibited at impulses greater than 600 ms and 10 V.

Thus, displays made with copolymer-coated copper chromite pigment inaccordance with the present invention can provide good image stabilityand have a number of advantages over inclusion of polymers in thesuspending fluid, especially in that the use of copolymer-coated pigmentdoes not sacrifice response time to achieve good image stability. Thisadvantage in response time can be used to operating a display at lowervoltage. The displays reported in Table 4 switch almost to saturation in300 ms at 7.5V, and achieve optimal contrast ratio at 10V and 250–300ms. At 15 volts, switching times of around 100 ms can be achieved.

It will be apparent to those skilled in the art that numerous changesand modifications can be made in the specific embodiments of the presentinvention described above without departing from the scope of theinvention. In particular, although the invention has been describedabove mainly in connection with encapsulated electrophoretic mediahaving discrete capsules, similar advantages can be achieved by the useof copper chromite pigments of the present invention in other types ofelectrophoretic media, for example unencapsulated electrophoretic media,polymer-dispersed electrophoretic media and microcell electrophoreticmedia. Accordingly, the whole of the foregoing description is to beconstrued in an illustrative and not in a limitative sense.

1. An electrophoretic medium comprising at least one electricallycharged particle suspended in a suspending fluid and capable of movingthrough the fluid on application of an electrical field to the fluid,wherein the at least one electrically charged particle comprises copperchromite.
 2. An electrophoretic medium according to claim 1 wherein theat least one electrically charged particle has an average diameter offrom about 0.25 to about 2 μm.
 3. An electrophoretic medium according toclaim 1 wherein the at least one electrically charged particle is coatedwith silica.
 4. An electrophoretic medium according to claim 3 whereinthe at least one particle has a polymer chemically bonded to the silicacoating.
 5. An electrophoretic medium according to claim 1 wherein theat least one electrically charged particle has a polymer chemicallybonded to, or cross-linked around, the at least one particle.
 6. Anelectrophoretic medium according to claim 5 wherein the polymer ischemically bonded to the at least one particle.
 7. An electrophoreticmedium according to claim 6 wherein the polymer comprises from about 5to about 500 mg m⁻² of the surface area of the at least one particle. 8.An electrophoretic medium according to claim 7 wherein the polymercomprises from about 10 to about 100 mg m⁻² of the surface area of theat least one particle.
 9. An electrophoretic medium according to claim 8wherein the polymer comprises from about 20 to about 100 mg m⁻² of thesurface area of the at least one particle.
 10. An electrophoretic mediumaccording to claim 9 wherein the polymer comprises amino groups.
 11. Anelectrophoretic medium according to claim 7 wherein the polymercomprises from about 2 to about 8 percent by weight of the at least oneparticle.
 12. An electrophoretic medium according to claim 6 wherein thepolymer comprises charged or chargeable groups.
 13. An electrophoreticmedium according to claim 6 wherein the polymer comprises a main chainand a plurality of side chains extending from the main chain, each ofthe side chains comprising at least about four carbon atoms.
 14. Anelectrophoretic medium according to claim 6 wherein the polymer isformed from an acrylate or a methacrylate.
 15. An electrophoretic mediumaccording to claim 6 wherein the polymer is bonded to the at least oneparticle via a residue of a functionalization agent.
 16. Anelectrophoretic medium according to claim 15 wherein thefunctionalization agent comprises a silane.
 17. An electrophoreticmedium according to claim 15 wherein the residue of thefunctionalization agent comprises charged or chargeable groups.
 18. Anelectrophoretic medium according to claim 1 further comprising at leastone second particle having at least one optical characteristic differingfrom that of the copper chromite particle(s), the at least one secondparticle also having an electrophoretic mobility differing from that ofthe copper chromite particle(s).
 19. An electrophoretic medium accordingto claim 18 wherein the copper chromite particle(s) and the secondparticle(s) bear charges of opposite polarity.
 20. An electrophoreticmedium according to claim 18 wherein the second particle(s) aresubstantially white.
 21. An electrophoretic medium according to claim 20wherein the second particle(s) comprise titania.
 22. An electrophoreticmedium according to claim 1 wherein the suspending fluid comprises ahydrocarbon, or a mixture of a hydrocarbon and a halogenatedhydrocarbon.
 23. An electrophoretic medium according to claim 1 furthercomprising a capsule wall within which the suspending fluid and the atleast one particle are retained.
 24. An electrophoretic medium accordingto claim 23 comprising a plurality of capsules each comprising a capsulewall and the suspending fluid and at least one particle retainedtherein, the medium further comprising a polymeric binder surroundingthe capsules.
 25. An electrophoretic display comprising anelectrophoretic medium according to claim 1 and at least one electrodedisposed adjacent the electrophoretic medium for applying an electricfield to the medium.
 26. An electrophoretic display according to claim25 wherein the electrophoretic medium comprises a plurality of capsules.27. An electrophoretic display according to claim 25 wherein theelectrophoretic medium comprises a plurality of droplets comprising thesuspending fluid and a continuous phase of a polymeric materialsurrounding the droplets.
 28. An electrophoretic display according toclaim 25 wherein the electrophoretic medium comprises a substrate havinga plurality of sealed cavities formed therein, and the suspending fluidand the copper chromite particles are retained within the sealedcavities.